Multiple innervation of human muscle fibers

A combination of electrophysiological techniques, including stimulus threshold measurements, coaxial needle recordings of voluntary EMG, and evoked response analysis, has been used to show that the motor innervation of the human biceps brachii has an extensive distribution in the long axis of the muscle. Impulse collision experiments, involving either excitation at two sites or the use of stimuli of graded intensity, have demonstrated that the diffuse muscle innervation is partly a consequence of individual muscle fibers having more than one end-plate     

Induction of the action potential in innervated slow muscle fibers of the frog. Effects of tetrodoxin, vincristine and colchicine

Experiments were made to induce action potentials in innervated slow muscle fibers of Rana temporaria. Drugs were applied to the sciatic nerve or its spinal roots by one of the following methods: implantation of a silastic cuff containing tetrodotoxin (TTX); subepineural injection of TTX combined with subcutaneous injections of vincristine; and epineural application of colchicine or vincristine. Twelve to 50 days later slow fibers responded to direct or indirect stimulation with an action potential. A substantial number of these fibers were innervated by 2 or 3 slow motor axons. It is concluded that these effects of TTX and colchicine (vincristine) are due to inhibition of the axoplasmic transport of an unidentified substance. The results support the hypothesis that the inexcitability of normal slow fibers is due to a 'trophic' effect of their motor axons.     

Increased jitter and blocking in normal muscles due to doubly innervated muscle fibers

Increased jitter and intermittent impulse blocking in electromyographic (EMG) signals are considered evidence of transmission abnormality and are not usually associated with normal muscle. However, motor unit action potentials (MUAPs) that exhibit increased jitter and blocking have recently been shown to occur in the brachioradialis muscles of neurologically healthy subjects. The jitter and blocking result from collisions, refractoriness, and conduction-velocity variability in long muscle fibers that are innervated by two different motoneurons at widely separated endplates. We analyzed MUAPs obtained by decomposing EMG signals from the brachioradialis muscles of four normal subjects. The rate of blocking of some MUAP components was as high as 28%, the jitter between some components exceeded 300 micros (mean consecutive difference), and the mean incidence of irregular MUAPs was 14%. These values would be considered abnormal in many other muscles. Jitter from doubly innervated fibers can be distinguished from other types of pathological jitter because one component exhibits amplitude variability. Clinical neurophysiologists should be aware that increased jitter and blocking do not necessarily indicate pathology in brachioradialis and perhaps other long, parallel-fibered muscles.     

自动分解肌电图的临床研究

探讨自动分解肌电图对神经肌肉疾病的诊断价值。方法：采用Ｎｉｃｏｌｅｔ和ＶｉｋｉｎｇＩＶ型肌电图仪,以常规同心圆针电极记录,对７１例健康成人和４３例神经肌肉疾病患者的肱二头肌或／和胫前肌进行检测,测定运动单位动作电位的５个参数;波幅、时限、转折数、发放率、棘波间隔变异系数。     

Electrophysiological characteristics of motor units and muscle fibers in trained and untrained young male subjects

We hypothesized that the amplitudes of compound muscle action potentials (CMAPs) and interference pattern analysis (IPA) would be larger in trained subjects compared with untrained subjects, possibly due to hypertrophy of muscle fibers and/or increased central drive. Moreover, we hypothesized that the untrained muscle is less excitable compared with the trained muscle. An electromyographic (EMG) needle electrode was used to record the IPA at maximal voluntary effort. The CMAP was obtained by stimulating the musculocutaneous nerve and recording the brachial biceps muscle using surface electrodes. CMAPs were obtained by direct muscle stimulation (DMS) with two stainless‐steel subdermal electrodes placed subcutaneously in the distal third of the muscle. Amplitudes of CMAP and IPA were significantly larger in trained subjects compared with untrained subjects. We found no differences between trained and untrained subjects in IPA power spectrum and turns per second or amplitude of the CMAPs obtained by DMS. Muscle fiber hypertrophy and/or altered central drive may account for our results, but there was no indication of changes in muscle fiber excitability. Muscle Nerve, 2010     

Compound muscle action potential cartography of an accessory peroneal nerve

In daily practice, accessory peroneal nerves (APNs) are detected in less than the 18–25% of legs, as revealed by systematic searches. In one APN case, compound muscle action potential cartography showed that the APN was only apparent when the recording electrode was placed over a small lateral region of the extensor digitorum brevis muscle. Effects of recording site can explain why many APNs go unrecognized. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:1331–1333, 1998.     

Motor unit number index examination in dominant and non-dominant hand muscles

This study investigated the effect of handedness on motor unit number index (MUNIX). Maximal hand strength, compound muscle action potential (CMAP) and voluntary surface electromyography (EMG) signals were measured bilaterally for the first dorsal interosseous (FDI) and thenar muscles in 24 right-handed and 2 left-handed healthy subjects. Mean (±standard error) grip and pinch forces in the dominant hand were 43.99 ± 2.36 kg and 9.36 ± 0.52 kg respectively, significantly larger than those in the non-dominant hand (grip: 41.37 ± 2.29 kg, p < .001; pinch: 8.79 ± 0.46 kg, p < .01). Examination of myoelectric parameters did not show a significant difference among the CMAP area, the MUNIX or motor unit size index (MUSIX) between the two sides in the FDI and thenar muscles. In addition, there was a lack of correlation between the strength and myoelectric parameters in regression analysis. However, strong correlations were observed between dominant and non-dominant hand muscles in both strength and myoelectric measures. Our results indicate that the population of motor units or spinal motor neurons as estimated from MUNIX may not be associated with handedness. Such findings help understand and interpret the MUNIX during its application for clinical or laboratory investigations.     

Comparison of single-fiber and macro electrode recordings: Relationship to motor unit action potential duration

The factors contributing to the duration of a motor unit action potential (MUAP) are believed to be well known, with both manual measurements and computer simulations agreeing with respect to MUAP durations approaching 10 ms. In this investigation, it is clearly demonstrated that use of a wide-open amplifier bandpass combined with signal-to-noise ratio enhancement results in MUAP durations approaching 30 ms recorded with either a macro or single-fiber electrode. Why the clinically recorded MUAP duration differs significantly from these physiologic durations is discussed. A hypothesis is presented whereby the major contributing factor toward MUAP duration is the total time of action potential transmembrane current flow along the muscle fiber from end-plate zone to musculotendinous junction. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1381–1388, 1997     

Origin of acetylcholinesterase in the neuromuscular junction formed in the in vitro innervated human muscle

Synaptic basal lamina is interposed between the pre- and postsynaptic membrane of the neuromuscular junction (NMJ). This position permits deposition of basal lamina-bound NMJ components of both neuronal and muscle fibre origin. One such molecule is acetylcholinesterase (AChE). The origin of NMJ AChE has been investigated previously as the answer would elucidate the relative contributions of muscle fibers and motor neurons to NMJ formation. However, in the experimental models used in prior investigations either the neuronal or muscular components of the NMJs were removed, or the NMJs were poorly differentiated. Therefore, the question of AChE origin in the intact and functional NMJ remains open. Here, we have approached this question using an in vitro model in which motor neurons, growing from embryonic rat spinal cord explants, form well differentiated NMJs with cultured human myotubes. By immunocytochemical staining with species-specific anti-AChE antibodies, we are able to differentiate between human (muscular) and rat (neuronal) AChE at the NMJ. We observed strong signal at the NMJ after staining with human AChE antibodies, which suggests a significant muscular AChE contribution. However, a weaker, but still clearly recognizable signal is observed after staining with rat AChE antibodies, suggesting a smaller fraction of AChE was derived from motor neurons. This is the first report demonstrating that both motor neuron and myotube contribute synaptic AChE under conditions where they interact with each other in the formation of an intact and functional NMJ.     

Differentiation between single fiber potentials from one muscle fiber or contaminated by other fibers using discriminating function

ObjectivesThe aim was to improve the identification of potentials recorded using single fiber electromyography (SFEMG) contaminated by potentials from other muscle fibers, which might affect measured jitter value, by defining more selective criteria of single fiber potential (SFP) discrimination. We were looking for solutions suitable for automatization.MethodsStandard parameters characterizing SFP and their combinations were analyzed to define an analytical discriminating function able to verify if potentials recorded using SFEMG are due to single fiber or due to two (or more) fibers.ResultsThe discriminating function is based on combination of standard SFP parameters. The procedure was tested on a set of simulated i.e., known data and on samples of clinical data. The tests on simulated data confirmed assumed properties of discriminating function. Preliminary results of pilot studies using patient data suggest its ability for differentiation between potentials of one fiber and contaminated ones. The procedure is suitable for automatization.ConclusionResults suggest that proposed discriminating function when supplementing standard criteria would help to promote SFP recordings and enable to improve relevancy of jitter measurements and of jitter value norms.     

Human motor unit activity during post-vibratory and imitative voluntary muscle contractions

Applying mechanical vibration for short periods to a muscle tendon induces long-lasting involuntary contractions which develop soon after the vibration offset in the previously vibrated muscle. In the present study, the question was raised as to whether these post-vibratory motor responses are mediated by the activity of supraspinal neural population or whether they may involve in addition some peripheral facilitatory influences operating at the motoneuronal level. To investigate this question, we analysed the unitary activity of 48 motor units belonging to the wrist extensor radialis muscles of the human hand, after attempting to classify them as slow or fast, during both post-vibratory and voluntary contractions having almost the same amplitudes and time-courses. The motor units were found to be activated in much the same way with both types of contraction. Similarities were observed as regards: the nature of the motor units activated, the order of recruitment of the motor unit population, the motor units' force recruitment thresholds, the mean interspike interval and the standard deviation. These analogies suggest that post-vibratory contraction may mainly involve a supraspinal tonic drive, but the possibility that these involuntary contractions may have a spinal origin cannot be completely ruled out.     

Fibromyalgia: Increased reactivity of the muscle membrane and a role of central regulation

ObjectiveFibromyalgia (FM) is characterized by widespread muscle pain and central neural deregulation. Previous studies showed increased muscle fiber conduction velocity (CV) in non-painful muscles of FM patients. This study investigates the relationship between central activation and the CV in FM.MethodsTwenty-two females with primary FM and 21 controls underwent surface electromyography of the non-painful biceps brachii. Mean CVs were calculated from the motor unit potential velocities (CV-MUPs), and the CV-MUPs’ statistical distributions were presented as histograms. The amount of muscle activity (average rectified voltage, ARV) was measured.ResultsThe CV was higher in the FM-group than in the controls (P = 0.021), with CV-MUPs generally shifted to higher values, indicative of increased muscle membrane propagation speeds. The largest increase in the CV of the FM-group occurred when adopting and maintaining a limb position at only 5% of maximum strength (P < 0.001); the CV did not, as normal, increase with greater force. However, the ARV in both groups similarly increased with force.ConclusionsIn fibromyalgia patients, the muscle membrane propagation speed increases independently of the force load or amount of muscle activity produced. When adopting a limb position, the patients show an augmented muscle membrane reaction, suggesting deregulation from higher neural centers.SignificanceThese findings contribute to understanding fibromyalgia.     

Anatomical and electrophysiological determinants of the human thenar compound muscle action potential

Clinical interpretation of the compound muscle action potential (CMAP) requires a precise understanding of its underlying mechanisms. We recorded normal thenar CMAPs and motor unit action potentials using different electrode configurations and different thumb positions. Computer simulations show that the CMAP has four parts: rising edge, negative phase, positive phase, and tail, which correspond to four distinct stages of electrical activity in the muscle: initiation at the end-plate, propagation, termination at the muscle/tendon junctions, and slow repolarization. The shapes of volume-conducted signals recorded beyond the muscle are also explained by these four stages. Changes in CMAP shape associated with thumb abduction are due to changes in termination times resulting from changes in muscle-fiber lengths. These findings demonstrate that the negative and positive phases of the CMAP are due to different mechanisms, and that anatomical factors, particularly muscle-fiber lengths, play an important role in determining CMAP shape. © 1996 John Wiley & Sons, Inc.     

Calcium sensitivity of fast- and slow-twitch human muscle fibers

Muscle fibers from the lateral gastrocnemius muscles of five normal adult males were chemically skinned (sarcolemma disrupted) and each fiber was divided into two parts for histochemical determination of fiber type and calcium-induced tension measurements. The calcium concentration associated with half-maximal tension was 2.5 microM for fast-twitch and 1.3-1.4 microM for slow-twitch fibers. Fast-oxidative-glycolytic (type 2A) and fast-glycolytic (type 2B) fibers had similar calcium sensitivities. Maximum tensions (kg/cm2) were 2.24 for fast-twitch and 2.19 for slow-twitch fibers.     

Histological study of motor innervation of nuclear bag1 intrafusal muscle fibers in the cat

The nerve supply to spindles of the cat tenuissimus muscle was reconstructed with light and electron microscopy of serial transverse sections. Fifty-two poles of the nuclear bag1 intrafusal muscle fiber were examined for motor innervation. The fiber poles were supplied by 71 myelinated motor axons that either terminated on bag1 fibers exclusively (93%) or coinnervated a chain fiber of the same intrafusal bundle (7%). No axons coinnervated both the bag1 and bag2 fibers. The unmyelinated preterminal segments of the axons were frequently short. Lengths and pre- and postsynaptic features of motor endings on bag1 fibers were variable. These features did not permit reliable classification of the endings into more than one morphological category. Moreover, the terminals of fusimotor (gamma) and skeletofusimotor (beta) axons on bag1 fibers appeared similar in cross-section. The degree of indentation of axon terminals into the surface of bag1 fibers increased with increasing distance from the spindle equator. However, cross-sectional areas of sole plates and axon terminals were relatively constant regardless of distance from the equator. The subjunctional membranes of both gamma and beta bag1 endings were typically smooth in contour. Bag1 endings differed from those on bag2 and typical chain fibers in having a thicker sole plate, frequently indented axon terminals, and unfolded subjunctional membranes. None of the bag1 endings resembled an extrafusal end plate. These observations indicated that (1) the dynamic (bag1) and static (bag2 and chain) intrafusal systems of the cat spindle are under separate motor control, and (2) the type of intrafusal fiber and the distance of the motor ending from the equator have a greater influence on the form and structure of bag1 endings than do supplying axons.     

Rippling muscle disease may be caused by "silent" action potentials in the tubular system of skeletal muscle fibers

Rippling muscle disease (RMD) is a generally benign, myotonic-like myopathy associated with rapid rolling contractions and percussion-induced contractions. These contractions are electrically silent in electromyographic recordings, which is taken as evidence that action potentials are not involved in the phenomena. The pathophysiological mechanisms underlying the symptoms have not been elucidated. Many cases of RMD are caused by mutations in caveolin-3, and aberrations in the tubular system are commonly observed. Here, recent data are discussed showing that action potentials can travel over substantial distances entirely within the transverse and longitudinal tubular systems of a muscle fiber and that stretch can induce such action potentials. Action potentials travelling in the tubular system in most circumstances probably cannot excite the sarcolemma and hence would not be detected. It is suggested that the distinctive contractions in RMD may be due to stretch-induced generation of action potentials within the tubular system.     

Electromyographic and vectorelectromyographic studies of potentials recorded from human single muscle fibers

Potentials from human single muscle fibers and their vectorelectromyographic image were studied. We recorded the potentials close to the motor end-plate, near the fiber ends, and beyond the fiber ends. We investigated the influence of the position of the electrode with respect to the muscle fiber and the length of the depolarized zone. We showed that the vectorelectromyographic image of the impulses from single muscle fibers may be used for correct orientation of the electrodes, for differentiation between potentials from single muscle fibers and composite potentials, and for tracing changes in the length of the depolarized zone which increases during long-lasting firing.     

Low-rate nerve stimulation during regional ischemia in the diagnosis of muscle glycogenosis

Five patients with muscle glycogenoses (Gly), 30 normal subjects (NS), and 52 disease controls received 3 Hz repetitive stimulation of the ulnar nerve for 4 min (P-LRNS) during regional ischemia and during normal circulation. During regional ischemia, the compound muscle action potential (CMAP) of NS showed 8.5% (SE: 1.0) facilitation after 1 min of P-LRNS and 4.8 ± 1.5% facilitation after 4 min. Gly showed 8.1 ± 1.0% facilitation after 1 min of P-LRNS but ?58.6 ± 6.9% depression after 4 min of P-LRNS. During normal circulation, in 18 NS and 4 Gly tested, the facilitation detected after 1 min of P-LRNS was unchanged until the end of stimulation. Compared with the mean ± 2 SD of NS, individual values of CMAP depression were abnormal in all Gly patients already by the 3rd min of ischemic P-LRNS. of disease controls, myasthenia gravis patients only showed a CMAP depression during ischemic P-LRNS which was distinguishable from that detected in Gly, being mainly induced by neuromuscular transmission block. © 1996 John Wiley & Sons, Inc.     

Ultrastructural study of motor endings on long chain intrafusal fibers in muscle spindles of the cat

Five complete poles of long nuclear chain intrafusal fibers of cat tenuissimus muscle spindles embedded in a plastic medium were cut in serial, 1 micron thick transverse sections and stained with toluidine blue. Each fiber pole displayed one plate-type motor ending positioned outside the encapsulated part of the spindle. Ultrastructurally, the endings resembled extrafusal end-plates and were more complex, in terms of prominence of sole-plate and degree of post-junctional folding, than any other motor ending present in the spindles. They were identified as the terminals of static (fast) skeletofusimotor axons, which preferentially innervate the longest nuclear chain fibers of cat spindles.